A Gas–Solid–Liquid Coupling Model of Coal Seams and the Optimization of Gas Drainage Boreholes

Abstract

For a gas-solid-liquid coupling model of coal seams, previous permeability models basically supposed uniaxial strains as the boundary condition for coal reservoirs without considering the deformation caused by changes in humidity. The permeability model varies under different boundary conditions. According to the true triaxial stress state of coal reservoirs, a permeability model considering the effective stress, sorption and desorption, and wet strain was established. Based on the permeability model, the continuity equation of gas and water and the stress field equation were coupled. Then, the model was incorporated in the COMSOL suite to simulate gas drainage from boreholes in floor roadways passing through seams in a coal mine. By comparing with the measured gas flow on site, the model was verified as being reliable. Moreover, the spacing and layout shape of boreholes in floor roadways were simulated. To achieve the aim of eliminating regional outburst within 180 days and decreasing the number of boreholes so as to reduce the cost, the spacing and shape of boreholes were optimized. When the superimposed effect of the boreholes was not considered, the optimal spacing of boreholes was √ 3r; if the superimposed effect was taken into account, the spacing could be set to within √ 3r ≤ L ≤ R, where r and R represent the effective gas drainage radius and the influence radius of gas drainage, respectively. The borehole spacing could be appropriately increased when the boreholes were arranged in rhomboidal form. To achieve the same range of outburst elimination, the rhomboidal layout can decrease the number of boreholes to reduce cost, thus realizing the objective of this optimization process.

Theoretical, numerical, and experimental analysis of effective extraction radius of coalbed methane boreholes by a gas seepage model based on defined criteria

“…Figure shows the critical gas pressure for these two criteria. The red line represents the first criterion, which was used in some literatures, while the blue line represents the second criterion, which was adopted in other literatures . The two lines intersect where the original gas pressure P 0 is 1.08 MPa.…”

“…If the gas extraction borehole spacing is too large, there will be extraction blind zones formed between extraction boreholes, bringing extremely large potential accidents to production in coal mines. If the gas extraction borehole spacing is too small, the gas extraction rate and gas extraction volume may be increased to a certain extent, but this will increase the borehole construction workload, resulting in an increase in the gas extraction cost . Therefore, the accurately determined effective extraction radius of coal seam boreholes is the key to increasing CBM production and reducing coal mine disasters.…”

“…The variation of gas pressure with the extraction or mining pressure relief is continuously observed until the gas pressure measurement is completed. When the measured gas pressure drops to a safe range, the effective extraction radius of boreholes can be obtained more accurately by the gas pressure method . In accordance with the Regulations on Prevention and Control of Coal and Gas Outburst, the effective extraction radius can be determined at the maximum gas pressure (0.74 MPa), the maximum gas content (8 m 3 /t), or the gas content decreased by more than 30% .…”

“…If the gas extraction borehole spacing is too small, the gas extraction rate and gas extraction volume may be increased to a certain extent, but this will increase the borehole construction workload, resulting in an increase in the gas extraction cost. [25][26][27] Therefore, the accurately determined effective extraction radius of coal seam boreholes is the key to increasing CBM production and reducing coal mine disasters.…”

“…Year Others Electromechanical Transportation Blasting Fire Gas Floor by the gas pressure method. 25,27,31,40,41 In accordance with the Regulations on Prevention and Control of Coal and Gas Outburst, the effective extraction radius can be determined at the maximum gas pressure (0.74 MPa), the maximum gas content (8 m 3 /t), or the gas content decreased by more than 30%. 42 However, most researchers only considered one or two of these criteria, not all the three criteria, to evaluate the effective extraction radius, which leads to inaccurate results.…”